MainPageViewModel.cs

namespace DynamicDataGroupingSample
{
    public class MainViewModel : ReactiveObject, IDisposable
    {
        public MainViewModel()
        {
            ...
            //Filter logic
            Func<Restaurant, bool> countryFilter(string country) => restaurant =>
            {
                return country == "All" || country == restaurant.Country;
            };

            var countryPredicate = this.WhenAnyValue(x => x.SelectedCountryFilter)
                                       .Select(countryFilter);
                                       
           _cleanUp = _sourceCache.Connect()
            .RefCount()
            .Filter(countryPredicate)
            .Filter(filterPredicate)
            .Sort(sortPredicate)
            .Bind(out _restaurants)
            .DisposeMany()
            .Subscribe();

        }
        private string SortBy
        {
            get => _sortBy;
            set => this.RaiseAndSetIfChanged(ref _sortBy, value);
        }
        
         private string _sortBy;
    }
}